The flow rate of data transmitted by telecommunications networks is continually increasing. Therefore, increasingly more optic fires are used in these networks to meet these high speed requirements.
For every optic fibre installed, a verification of its characteristics must be made to ensure that they meet specifications and have no breaks or major attenuation.
The most frequently used devices to conduct these verification operations are devices called Optical Time-Domain Reflectometers (OTDRs). Optical reflectometry is also used by the operator to detect the position of a fault for faster, more efficient network repairs.
The principle of the OTDR technique is the detection and analysis, as a function of time, of the light backscattered by small imperfections and impurities present in the fibre (phenomenon known as Rayleigh back-scattering) and of the light reflected within the fibre (reflection on connectors, splices . . . ). The method consists of sending out a short impulse, from one end of the fibre, which propagates along the fibre and of measuring the quantity of light, as a function of time, which is backscattered towards a detector. On account of small imperfections and impurities in the fibre, part of the light is scattered in all directions. An ultrasensitive detector measures the quantity of backscattered light, i.e. which moves in the opposite direction to the direction of the incident impulse. With knowledge of the quantity of light that is at all times backscattered towards the detector, it is possible to determine the distribution of losses in the optic fibre. Therefore, a loss or fault at a determined point of the fibre will give rise to transitory discontinuity in the backscattered Optical Power tracing.
In tree topology networks, as in point-to-point networks, the OTDR system must be able to precisely locate faults occurring on the line. But this operation is made difficult on a tree-type network since all backscattered signals of all the lines are added together. Even if it is always easy to measure the distance between the fault and the OTDR, it is much more difficult to determine on which lines the faults have occurred.
One solution consists of placing a selective mirror at the end of the line of each subscriber, said mirror reflecting a predetermined wavelength, 1625 nm for example. Each subscriber is at a different distance from the detector of the OTDR system. The presence of the mirror leads to the presence of a reflective peak on the backscattered Optical Power tracing. The presence of a fault translates into transitory discontinuity in the monotonicity of the backscattered Optical Power tracing, indicating the distance between the OTDR and the fault. But with this indication alone, the fault may be located on any of the fibres of the tree network. It is the reflective peak which enables determination of the fibre on which the fault is located. Said reflective peak, highly attenuated or inexistent, indicates the presence of a fault on the line with which the mirror is associated. This line can be identified since each mirror is at a different distance from the OTDR.
However, the use of this kind of solution raises certain difficulties.
Firstly one must make sure that each mirror is at a different distance, otherwise the reflective peaks will be confused and it will no longer be possible to make a distinction between two branches. This is not an easy condition to meet, since it is difficult to know the exact length of the fibre on account of all the overlengths stored in the bays or boxes.
Also, when a fault occurs on two different lines, the faults are observed on the OTDR display. Two transitory discontinuities in the monotonicity of the Optical Power tracing are visible. These two faults will also lead to the presence of two attenuated peaks derived from the reflection of the two mirrors associated with each of the two faulty lines. The presence of the two peaks enables identification of the two faulty lines. However, it is difficult to allocate each of the two transitory discontinuities in the monotonicity of the tracing to the faulty line associated with it, and hence to determine the location of the fault on the faulty line. This difficulty in locating the fault in the event of several faulty lines is all the more critical since the losses on each of the two lines are substantially the same.